Chapter 5: Repetition Statements

Transcription

1 Chapter 5: Repetition Statements 1

2 Objectives In this chapter, you will learn about: Basic loop structures while loops Interactive while loops for loops Loop programming techniques 2

3 Objectives (continued) Nested loops do while loops Common programming errors 3

4 Basic Loop Structures Repetition structure has four required elements: Repetition statement Condition to be evaluated Initial value for the condition Loop termination Repetition statements include: while for do while 4

5 Basic Loop Structures (continued) The condition can be tested At the beginning: Pretest or entrance-controlled loop At the end: Posttest or exit-controlled loop Something in the loop body must cause the condition to change, to avoid an infinite loop, which never terminates 5

6 Pretest and Posttest Loops Pretest loop: Condition is tested first; if false, statements in the loop body are never executed while and for loops are pretest loops Figure 5.1 A pretest loop 6

7 Pretest and Posttest Loops (continued) Posttest loop: Condition is tested after the loop body statements are executed; loop body always executes at least once do while is a posttest loop Figure 5.2 A posttest loop 7

8 Fixed-Count Versus Variable-Condition Loops Fixed-count loop: Loop is processed for a fixed number of repetitions Variable-condition loop: Number of repetitions depends on the value of a variable 8

9 while Loops while statement is used to create a while loop Syntax: while (expression) statement; Statements following the expressions are executed as long as the expression condition remains true (evaluates to a non-zero value)!! 9

10 while Loops (continued) 10

11 Interactive while Loops Combining interactive data entry with the while statement provides for repetitive entry and accumulation of totals! 11

12 Interactive while Loops (cont d) Figure 5.7 Accumulation flow of control 12

13 Sentinels Sentinel: A data value used to signal either the start or end of a data series Use a sentinel when you don t know how many values need to be entered! 13

14 break and continue Statements break statement Forces an immediate break, or exit, from switch, while, for, and do-while statements Violates pure structured programming, but is useful for breaking out of loops when an unusual condition is detected 14

15 break and continue Statements (cont d) Example of a break statement: 15

16 break and continue Statements (cont d) A continue statement where invalid grades are ignored, and only valid grades are added to the total: 16

17 break and continue Statements (cont d) continue statement Applies to while, do-while, and for statements; causes the next iteration of the loop to begin immediately Useful for skipping over data that should not be processed in this iteration, while staying within the loop 17

18 The Null Statement Null statement Semicolon with nothing preceding it ; Do-nothing statement required for syntax purposes only 18

19 for Loops for statement: A loop with a fixed count condition that handles alteration of the condition Syntax: for (initializing list; expression; altering list) statement; Initializing list: Sets the starting value of a counter Expression: Contains the maximum or minimum value the counter can have; determines when the loop is finished 19

20 for Loops (continued) Altering list: Provides the increment value that is added or subtracted from the counter in each iteration of the loop If initializing list is missing, the counter initial value must be provided prior to entering the for loop If altering list is missing, the counter must be altered in the loop body Omitting the expression will result in an infinite loop 20

21 for Loops (continued) 21

22 for Loops (cont d) Figure 5.10 for loop flowchart. 22

23 A Closer Look: Loop Programming Techniques These techniques are suitable for pretest loops (for and while): Interactive input within a loop Includes a cin statement within a while or for loop Selection within a loop Using a for or while loop to cycle through a set of values to select those values that meet some criteria 23

24 A Closer Look: Loop Programming Techniques (continued) 24

25 A Closer Look: Loop Programming Techniques (continued) 25

26 A Closer Look: Loop Programming Techniques (continued) Evaluating functions of one variable Used for functions that must be evaluated over a range of values Noninteger increment values can be used 26

27 A Closer Look: Loop Programming Techniques (continued) 27

28 A Closer Look: Loop Programming Techniques (continued) Interactive loop control Variable is used to control the loop repetitions Provides more flexibility at run-time Random numbers and simulation Pseudorandom generator used for simulators C++ functions: rand(); srand() 28

29 A Closer Look: Loop Programming Techniques (continued) 29

30 A Closer Look: Loop Programming Techniques (continued) 30

31 Nested Loops Nested loop: A loop contained within another loop All statements of the inner loop must be completely contained within the outer loop; no overlap allowed Different variables must be used to control each loop For each single iteration of the outer loop, the inner loop runs through all of its iterations 31

32 Nested Loops (continued) Figure 5.12 For each i, j loops. 32

33 Nested Loops (continued) 33

34 do while Loops do while loop is a posttest loop Loop continues while the condition is true Condition is tested at the end of the loop Syntax: do statement; while (expression); All statements are executed at least once in a posttest loop 34

35 do while Loops Figure 5.13 The do while loop structure. 35

36 do while Loops Figure 5.14 The do statement s flow of control. 36

37 Validity Checks Useful in filtering user-entered input and providing data validation checks!!!! Can enhance with if-else statement 37

38 Common Programming Errors Making the off by one error: loop executes one too many or one too few times Using the assignment operator (=) instead of the equality comparison operator (==) in the condition expression Testing for equality with floating-point or double-precision operands; use an epsilon value instead 38

39 Common Programming Errors (continued) Placing a semicolon at the end of the for clause, which produces a null loop body Using commas instead of semicolons to separate items in the for statement Changing the value of the control variable Omitting the final semicolon in a do statement 39

40 Summary Loop: A section of repeating code, whose repetitions are controlled by testing a condition Three types of loops: while for do while Pretest loop: Condition is tested at beginning of loop; loop body may not ever execute; ex., while, for loops 40

41 Summary (continued) Posttest loop: Condition is tested at end of loop; loop body executes at least once; ex., do while Fixed-count loop: Number of repetitions is set in the loop condition Variable-condition loop: Number of repetitions is controlled by the value of a variable! 41